The UN has warned about an upsurge in violence in Sudan's Darfur region, saying some 50,000 people had been displaced since the end of February.
Peacekeepers and aid agencies had been blocked from entering affected areas, UN human rights chief Navi Pillay said.
Much of the violence is between rival Arab groups, although rebel and government forces are also involved, a BBC correspondent reports.
Darfur has been hit by conflict since 2003, when rebels took up arms.
Two million people have already been displaced by the conflict.
The International Criminal Court has issued an arrest warrant for Sudan's President Omar al-Bashir, accusing him of committing war crimes and genocide against black African communities in Darfur.
He denies the charge.
On Monday, UN Secretary-General Ban Ki-moon said was deeply concerned about the escalating violence in Darfur, and he urged all parties to immediately stop hostilities.
Ms Pillay said some 45 villages were reportedly targeted in the Um Gunya area, about 250km (155 miles) south of Fasher, the capital of North Darfur state.
"I urge the authorities to protect civilians and hold to account those who have committed grave breaches of human rights and humanitarian laws," Ms Pillay said.
The joint UN and African Union (AU) peace force in Darfur, known as Unamid, and humanitarian agencies had been prevented from reaching areas affected by the attacks, she added.
"The Sudanese government must allow Unamid to fulfil its mandate to protect civilians, and grant access to populations in need," Ms Pillay said.
In the past week, Unamid had also reported looting and destruction in Saraf Omra, near the border with Chad, says BBC Africa Security correspondent Moses Rono.
Thousands of people uprooted by the inter-communal fighting have sought refuge near a Unamid compound, he adds.
The leader of Arab militiamen accused of fuelling conflict in Darfur, Musa Hilal, recently quit the government and threatened war, adding a new dimension to the conflict, our correspondent says.
The black African rebels took up arms in 2003, accusing the Arab dominated government in Khartoum of ignoring them.